1. Field of the Invention
The present invention relates to a pressure sensor. More specifically, it relates to a pressure sensor for converting a fluid pressure into an electronic signal to be outputted to an outside.
2. Description of the Related Art
[Outline of Pressure Sensor]
A pressure sensor that detects a difference between detected pressure and atmospheric pressure and converts into an electric signal is used for measuring a fluid pressure.
An example of the pressure sensor is shown in FIG. 24. In the FIG. 24, the pressure sensor 1A has a fitting 10 to be bolted and fixed to an attachment portion, a pressure-detecting module 20 to be attached to a weld 10A of the fitting 10 by beam-welding etc. and an output device 30 to be electrically connected to the pressure-detecting module 20. The fluid pressure is introduced into a pressure port 11 of the fitting 10 and is converted into a strain of a diaphragm 21 forming the pressure-detecting module 20. The strain is detected by a strain gauge (not shown) on the diaphragm 21 and the electric signal in accordance with a resistance value of the strain gauge is outputted by the output device 30.
The output device 30 has a base member 32 disposed around the pressure-detecting module 20, a circuit substrate (not shown) disposed above the base member 32, electric circuit components 50 such as an IC, resistor and capacitor to be installed on the circuit substrate for processing a signal from the pressure-detecting module 20 and an output connector (not shown) for taking out an output signal from the electric circuit components 50.
The pressure-detecting module 20 and the circuit substrate of the output device 30 is covered by a metal case 31 for electromagnetic shielding. A resin-made covering member (not shown) is provided outside the case 31 for the purpose of blocking dust etc. and forming an exterior of the pressure sensor.
[Deterioration of Soldered Portion]
The pressure-detecting module 20, circuit substrate and output connector of the output device are interconnected directly with respective connecting terminals or through cables. They are connected by inserting an end of the terminal or the cable to a through-hole of the circuit substrate and an output terminal and soldering the surroundings.
In some cases, the circuit substrate is composed of a plurality of plates spaced apart at a predetermined distance. In this case, terminals connecting the respective circuits are used. The terminals between the circuit substrates are generally used in plural, and are usually soldered to the opening of the circuit substrate with an end being inserted thereto while the other end is fixed to the other circuit substrate by soldering and the like.
However, when the pressure sensor is attached to a position where ambient temperature gets extremely high, such as a space adjacent to an engine room of an automobile etc., each component is thermally expanded due to the heat. Especially, if the terminal connecting respective member is thermally expanded, stress is concentrated to the soldered part having less strength, thereby deteriorating the end of the soldered terminal.
[Connection of the Pressure-detecting module and the Circuit Substrate]
The aforesaid pressure-detecting module 20 and the circuit substrate is connected as mentioned below. The circuit substrate has a circular configuration with an opening at the center thereof and the pressure-detecting module 20 is positioned in the opening of the circuit substrate so that the diaphragm 21 and the circuit substrate are approximately coplanar, where the strain gauge on the diaphragm 21 and a pad on the circuit substrate is electrically connected by wire-bonding and the like.
However, according to the aforesaid connection structure, the electric circuit components 50 such as IC, capacitor and resistor can not be installed to the opening of the circuit substrate and the pad for bonding. Accordingly, it is necessary to make the circuit substrate larger correspondingly to the area of the opening to secure sufficient installing area. Additionally, a diameter of the entire pressure sensor body get larger, so that a box wrench for attaching the pressure sensor can not be used in a certain circumambient configuration and a space of the attachment portion, thereby making attachment process difficult.
On the other hand, some of the pressure sensors as shown in Japanese Utility Model Application No. Hei-6-2189, for instance, has a space between the pressure-detecting module 20 and the circuit substrate and an L-shaped terminal projecting from the circuit substrate to the pressure-detecting module 20 to be electrically connected to the strain gauge by wire-bonding. Since the pressure-detecting module 20 and the circuit substrate are alienated and the circuit substrate has no opening, the electric circuit components can be installed all over both surface areas of the circuit substrate, thereby lessening the diameter of the circuit substrate. Accordingly, the diameter of the pressure sensor itself can be decreased to be adjusted to a narrower space.
However, though an end of the L-shaped terminal of the above connection structure is fixed to the circuit substrate, the other end, i.e. wiring side of the wire-bonding, is positioned between the circuit substrate and the pressure-detecting module 20 hanging in midair. So, in an ordinary bonding where an ultrasonic vibration is applied to a wire such as a gold wiring, the wired portion of the terminal can also be vibrated to deteriorate a bonding strength between the terminal and the wire.
[Electromagnetic Shield by a Metal Case]
The case 31 is a electromagnetic shielding member provided as a body independent to the covering member forming the exterior of the pressure sensor 1A, the case 31 being made of metal to protect securely the electric circuit components 50 from electromagnetic interference.
The covering member may be made of metal as an alternative way for protecting the electric circuit components 50. However, since the covering member is relatively a large member in the pressure sensor 1A, the weight reduction of the entire pressure sensor can be made difficult. Furthermore, the covering member has to be a die-casting molding or, alternatively, troublesome machine process is necessary for producing complicated configuration. Therefore, the cost becomes expensive.
Further alternatively, an electromagnetic shielding layer may be provided inside a resin-made covering member by coating a conductive paint. However, though weight of the pressure sensor 1A can be reduced in this structure, coating the conductive paint can be still expensive and reliability of a conductive portion to the fitting can be lowered by an abrasion of the electromagnetic layer.
Accordingly, the metal case 31 provided independently to the covering member allows the covering member to be made of resin to overcome above disadvantages, so that an anti-noise property of the pressure sensor 1A can be improved, weight can be reduced, production cost can be lowered and reliability can be enhanced.
[Problem in Locating the Case]
The case 31 of the pressure sensor shown in FIG. 24 is merely disposed over the fitting 10 with the pressure-detecting module 20 being inserted in the opening provided at a bottom. Accordingly, the position of the case 31 is likely to be displaced relatively to the fitting 10, and a locating work in welding the case 31 to the fitting 10 can be troublesome.
[Sealing by Resin-made Cover]
The covering member is made of resin in view of weight reduction and processing facilitation. The covering member and the fitting are interconnected by crimping the fitting and sealability thereof is improved by putting an O-ring etc. between the covering member and the fitting.
However, when the fitting is made of metal and the covering member is made of resin, the covering member may be separated from the crimped part of the fitting according to a shrinking degree, especially toward inside, of the covering member and the sealability is lost, since a thermal expansion coefficient is distinctively different between the metal and the resin.
An urethane resin, epoxy resin etc. is sometimes filled inside the covering member to restrain the shrinkage of the covering member. However, the filling process and solidification of the resin can take long time, so that productivity can be impaired.